Mirror Cleaning

Steward Observatory on Mount Lemmon

This procedure for cleaning large mirrors was demonstrated to us by Joseph Hochscheidt when he cleaned the Manner Telescope primary and secondary in June 2018. Use it with caution since this is based on notes taken at the time. Obviously the first rule is do no harm, and that means be reasonable, gentle, and follow the advice of experts when it is available.

For this cleaning process the mirror was left in the telescope. It was a warm sunny day, and the the telescope enclosure was opened slightly to allow for ventilation and to provide illumination needed to judge the effectiveness of the process.

1. Blow off all dust and loose particles with dry nitrogen gas from a compressed gas cylinder.

This requires bringing a cylinder to the telescope, attaching a regulator, and using a flexible hose that provides enough gas flow for the purpose. Do not use compressed air! It will contain moisture that will condense on the optics, and it may also contain fine particulates that could damage the coatings. An alternative that is often used with large optics is to use gas withdrawal carbon dioxide. This is available in small cylinders, and is released at full pressure through a hose to direct the flow onto the mirror and its shrouds. Carbon dioxide cleaning will produce CO₂ snow which can effectively scrub a mirror of dust. However it will be insufficient to remove deposits left by evaporating dew or by insects.

Assure that all loose material is blown out or wiped from the housing that baffles the mirror.

2. With the telescope pointed approximately horizontally, use large quantities of Kimwipes or paper towels rolled to absorb any fluids that drain from the mirror. These may be replaced several times during the cleaning process. The purpose is to prevent fluids from draining into critical components behind the primary, or auxiliary instruments or components that may be on the lower side of the truss. Each case will be unique and should be evaluated before beginning. It may be beneficial to point the telescope slightly below the horizontal if the light baffle around the primary is solid and would hold liquid that could drain to an area which is not critical.

It is not advisable to remove the primary. This adds considerable risk to the cleaning process, and would surely require telescope realignment. Also, primary removal would normally be preceded by mechanical work to assure the telescope remains stable, and would add days to a job that can be done in hours.